The use of liquid crystal displays to detect and indicate voltage presence is known from numerous solutions. By way of example, U.S. Pat. No. 4,112,361 reveals a liquid crystal voltmeter, in which a nematic liquid crystal is placed between a pair of semitransparent electrodes, which are deposited on the surfaces of polarizing substrate plates. One electrode is a thin resistive layer and the other electrode is a conducting layer. A source of direct-current (DC) voltage is connected to both ends of the resistive electrode and the conducting electrode is grounded. On the surface of one of the polarizing substrate plates there is placed a suitable scale indicating the value of the measured voltage that occurs between the electrode with resistive film and the ground.
A contactless electrostatic voltage sensor, which detects voltage that occurs on electrically charged surfaces is known from another U.S. Pat. No. 4,786,858. The sensor comprises a liquid crystal cell with two electrodes, of which one is electrically coupled through a capacitor having a mechanically controlled diaphragm with the charged surface whose voltage is to be measured, and the other is grounded. The diaphragm is connected to a reference voltage source. In addition, charging voltage is connected directly to the first electrode of the liquid crystal cell in order to charge the cell to a predetermined voltage.
The presented devices cannot be used to detect and/or indicate the presence of alternating-current (AC) voltage in medium and high voltage systems. In particular, the device described in the U.S. Pat. No. 4,112,361 is applicable mainly to measurement of DC voltages not higher than the threshold voltage of the used liquid crystal cell. AC voltage measurement is possible with the described device only when a reference AC voltage which is phase-compatible with the measured voltage is supplied. The need to supply such a reference voltage in the case of medium and high voltage systems would present a serious difficulty.
On the other hand, the device described in the U.S. Pat. No. 4,786,858 finds application in measuring DC voltage generated by an electrostatic charge accumulated on photosensitive elements of electrophotographic devices, thus it is not intended for measuring AC voltage. Moreover, in medium and high voltage equipment, electrostatic charges accumulate on the surface of conductors in an uncontrolled manner and a measurement made by such a device would be useless.
Liquid crystal cells and displays of various types show direct sensitivity to AC voltage and electric field. When AC voltage exceeding a given threshold value is applied to the electrodes of a liquid crystal cell, it generates an electrostatic field inside the cell of an intensity which changes its optical properties. Application of such voltage is also connected with the flow through the cell of electric current of intensity depending on the value of the applied voltage and the resistive and capacitive characteristics of the cell. In liquid crystal displays, a suitable shape of the electrodes makes it possible to apply voltage to localized cell fragments and to obtain visible contrast.
For typical liquid crystal cells of the type TN (twisted nematic), the AC voltage threshold value required to obtain the maximum optical effect is approximately 2V RMS (RMS—root-mean-square) at a frequency of approximately 50 Hz. At this threshold voltage, a current per surface area unit of approximately 1.5 μA/cm2 RMS flows through the cell. For typical cells of the type PDLC (polymer-dispersed liquid crystal) the intensity of the corresponding threshold current is approximately 15 μA/cm2 RMS per surface area unit at a 50 Hz frequency.
The existence of the threshold values of the AC voltage for liquid crystal cells and displays makes it possible to use them for contactless detection AC voltages. In particular, in the vicinity of every conductor connected to AC voltage there is an AC electric field, which also generates the flow of an electric current through the surface of the conductor. Such capacitive current flows in the direction parallel to the direction of the electric field, and the RMS intensity of this current I, measured per unit of surface area S, which is perpendicular to the direction of the capacitive current flow, is:I/S=2πv∈0∈E, where v is the frequency, ∈0 is the vacuum electric permittivity, ∈ is the dielectric constant of the material, and E is the RMS value of the electric field intensity.
Such a capacitive current can be used to indicate the presence of voltage being its source, by using it directly to energize a liquid crystal cell or display, and a change in the optical properties of the cell or display indicates the presence of a voltage exceeding a specific threshold.
In order to use the capacitive current associated with the electric field arising around live conductors to energize the indicator, a current-collecting electrode in the form of a flat conducting element placed in a plane perpendicular to direction of the electric field is typically used. The surface area of the current-collecting electrode must be sufficient to energize the liquid crystal elements activated when indicating voltage and to ensure a sufficient sensitivity of the indicator.
The current-collecting electrodes that are positioned parallel to the direction of the electric field can also energize the indicator. However, such positioning of the electrodes causes a local considerable increase in the electric field and may cause discharges, which have and adverse effect on electric power equipment.
A display detecting live wires is known from published Japanese patent application No. 61-003069. This device is intended to detect electric field near live conductors by using the threshold voltage of a liquid crystal display. A known two-electrode liquid crystal display (LCD) is provided with two additional electrodes, of which one is fixed to the front side of the display, and the other is fixed to its rear side, where side is understood to be this one which is placed on the surface of the object that is to be tested for the presence of electric field in its vicinity. Both electrodes of the display are electrically connected with the additional electrodes in such a way that each additional electrode is connected with a different electrode of the LCD. Due to the additional electrodes, the potential difference between the two electrodes of the liquid crystal element of the display, generated by the electric field of the tested object exceeds the display threshold value. When the tested object is live, the display shows the presence of voltage, which can be seen in the display window. In the presented solution, the device sensitivity sufficient to detect voltage presence was obtained by connecting additional external collecting electrodes to this display.
Another display used to indicate voltage presence is known from published Japanese description No. 63044173. Similarly to the device presented in description JP 61-003069, this device comprises a liquid crystal display and two additional current-collecting electrodes, of which one is galvanically connected with one, sign electrode of the display, whereas the other, which is transparent, is placed directly on the surface of the display and thereby capacitively coupled with the other, common electrode of the display. In this solution, the sufficient sensitivity of the device is achieved by providing an adequately large surface area of the additional current-collecting electrodes in relation to the surface area of the sign electrode of the display.
U.S. Pat. No. 4,818,072 and U.S. Pat. No. 4,838,653, both filed by Raychem Corporation, present a device for detecting voltage in electric cables or equipment and/or for detecting electric field generated in the vicinity of an electric device or equipment. The device has the form of a liquid crystal display, which is a liquid crystal cell whose electrodes are shaped as flat segments electrically separated from each other. Those electrodes are deposited on two substrate layers parallel to one another, and they are arranged so that a fragment of each segment deposited on one substrate layer overlaps with at least one fragment of the segment deposited on the other substrate layer. The overlapping segment fragments from both substrate layers form liquid crystal elements of the display, which are electrically connected with each other in series. The first liquid crystal element in non-energized state has electric capacitance lower than the capacitance of each other liquid crystal element of this cell, and the second liquid crystal element connected in series with the first liquid crystal element has basically the same size as the first element, and its capacitance in non-energized state is lower than the capacitance of the first liquid crystal element in energized state. In this solution, the connection of liquid crystal elements in series makes it possible to activate a given surface area of the display with electric current lower than in the solutions presented in the Japanese descriptions Nos. 61-003069 and 63-044173. Despite this, due to the small surface area of the conducting segments, the sensitivity of the device to the electric field is small, especially when the display is positioned in the plane perpendicular to the direction of the electric field. When the display is positioned in the plane parallel to direction of electric field, the sensitivity of the indicator increases, but such placement is not recommended for use with electric power equipment, because of the possibility of generating discharges.
A photoelectric sensor of electric field is known from published Japanese description No. 10260212. It comprises a liquid crystal located between overlapping electrodes, capacitively connected in series. It also comprises dipole antennas, which are electrically connected with the electrodes. In this solution, the sufficient sensitivity of the device is achieved by connecting additional dipole antennas of sufficiently large size to the LCD.
A disadvantage of all those presented solutions, when they are applied to electric power equipment, is that obtaining sufficient sensitivity of the device requires either attaching to the LCD additional external elements, which are much larger than the display itself, or considerably increasing the size of the display in relation to the size of the surface activated in the display during the indication of voltage or an electric field, or positioning the display in a plane parallel to the direction of the electric field, which can be a cause of discharges.